Problem: 2 notebooks cost $3.34. Which equation would help determine the cost of 7 notebooks?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 2 notebooks. We want to know the cost of 7 notebooks. We can write the numbers of notebooks as a proportion: $\dfrac{2}{7}$ We know 2 notebooks costs $3.34. We can let $x$ represent the unknown cost of 7 notebooks. The proportion of these costs can be expressed as: $\dfrac{\$3.34}{x}$ The cost changes along with the number of notebooks purchased, and so the two proportions are equivalent. $\dfrac{2}{7} = \dfrac{\$3.34}{x}$